When it comes to reader sensitivities about problematic words, I almost always err on the side of extreme caution. The phrase "illegal immigrant" was immediately changed to "undocumented immigrant" when a reader emailed with concerns. When someone thought our use of the word "Tribe" was "problematic and franky racist", I went straight to the source to investigate. Issues of cultural appropriation are always addressed head-on. But DERP? I'm not sure I agree that Derp is ableist.
Looking at the meme's history, it's just not a word with an etymology like "retarded" or "gay," where a descriptor has been lifted from a group of people and used to describe something negatively. (I'd feel quite differently about the word if its source was an evolution of, say, Flerderpington's Syndrome.) Derp is a word that was invented by a couple comedians to describe themselves doing something stupid. I'm not saying some awful people haven't used the word in mean ways (and I understand that some folks feel this is enough of a reason to avoid the word) but I'm just not seeing it as a word with the same troubling lineage as many other derisive phrases.
I'm not totally comfortable defending a phrase that some people find problematic, and I fully accept that I'm potentially wrong about this.
UPDATE 2 YEARS AFTER THIS POST WAS WRITTEN
Despite the fact that it felt like a made up phrase that (unlike many slurs and derisive words) doesn't target any specific group, any more than "D'OH!" in the '90s or "DUH" in the '80s… Ultimately for me, knowing people are concerned with it has been enough to make me stop using it on the websites — even if I'm not 100% sure I agree with the concern. Also, there are a lot more ridiculous words without any controversy that we can use. This post was helpful to me in coming to this conclusion:
There are other words, more descriptive words, and most people are capable of coming up with those words. For those without the mental wherewithal to tap into a good vocabulary, there's always a dictionary or thesaurus (look it up on your phone!).